Optical communications networks are becoming increasingly popular for data transmission due to their high bandwidth data transmission. Typically, a digital data stream is encoded (e.g. using On-Off-Keying—OOK) to generate sequential symbols that are conveyed through a communications channel by a respective optical channel signal. At a receiving end of the communications channel, an Optical-to-Electrical (O/E) converter detects the received optical channel signal and generates a corresponding analog electrical channel signal. The O/E converter is followed by an Analog-to-Digital (A/D) converter, which generates sequential N-bit samples (where N is at least one, and typically between 4 and 8), each of which is indicative of the detected power of the received channel signal at a particular instant. As such, the samples generated by the A/D converter reflect the combined effects of the encoded symbol values, attenuation, noise and any other signal distortions affecting the channel signal during transmission through the communications network. Thus, at best, the samples generated by the A/D converter represent a corrupted version of the original symbol values.
Various known decoding strategies may be used to process the “raw” samples generated by the A/D converter to detect and decode the symbols conveyed by the channel signal, and thereby recover the original digital data stream. For example, a digital equalizer may be used to process the samples generated by the A/D converter, in order to reduce inter-symbol interference (ISI). A Forward Error Correction (FEC) circuit may then be used to process the equalizer output to determine and generate the correct value of each bit of the recovered digital data stream.
As the traffic on fiber optic networks increases, monitoring and management of the networks become increasingly important issues. To monitor the network, the spectral characteristics of the composite signal at particular points in the network must be determined and analyzed. This information may then be used to optimize the performance of the network.
Ideally, performance monitoring of an optical communications system should be based on the analysis of the actual optical signal waveform, across the entire range of wavelengths of interest. Spectral analysis of this type can be performed using a variety of known signal and spectrum analysis equipment. For example, optical signal analyzers are known for determining characteristics of an optical signal such as, for example, power level, extinction ratio, eye opening, signal-to-noise ratio, polarization dependent loss (PDL), dispersion etc. In order to monitor respective channels of a Wavelength Division Multiplexed (WDM) communications system, either multiple signal analyzers can be arranged in parallel, or a single signal analyzer can be sequentially tuned to receive each optical channel signal in turn. Optical Spectrum Analyzers (OSAs) can be used to determine average and peak power levels, as a function of wavelength, across any desired range of wavelengths. This data may be used to monitor distributed gain and Raman scattering effects within the wavelength band of interest.
Due to their cost and complexity, conventional optical signal and spectrum analysis equipment is typically restricted to laboratory use. In order to monitor performance of installed optical communications systems, simpler and less expensive monitoring equipment is required. Typically, this simplified equipment relies on a low frequency pilot tone (typically on the order of 1 MHz or less) that is imposed on the optical signal at a transmitter end of an optical fiber. The residual pilot tone can then be detected at a desired monitoring point, and compared with the known parameters of the original pilot tone to estimate a performance parameter of the optical communications system. Suitable detectors can be installed on each channel of a WDM communications system to enable calculation of performance parameter values across a wavelength band of interest. In some cases, processing of per channel measurements can be used to estimate inter-channel effects. In some advanced optical communications systems, simplified analog Optical Spectrum Analyzers (OSAs) can be used to measure optical power as a function of wavelength. OSAs of this type are typically used in conjunction with optical amplifiers, in order to facilitate control of pump laser power levels. Typical optical performance monitoring systems known in the art are disclosed in co-assigned U.S. Pat. Nos. 5,513,024; 5,949,560; 5,999,258; 6,128,111; 6,222,652; and 6,252,692.
While the above-described systems enable some degree of performance monitoring, they tend to suffer a number of disadvantages. In particular, per-channel monitoring systems are typically dependent on a low frequency pilot tone (or dither) having known parameters. Any error between the design and actual parameter values of the launched pilot tone will naturally degrade the accuracy of any performance parameters calculated at the monitoring point. Additionally, this approach assumes that performance parameters calculated on the basis of the low frequency pilot tone will be valid for the high-speed data traffic. Consequently, any frequency-dependent effects cannot be detected (or compensated) with this arrangement. Finally, the detectors and signal processors utilized in these monitoring systems are low frequency analog devices. This facilitates real-time calculation of performance parameters using low cost devices. However, this is obtained at a cost of versatility, and limits the scope of analysis applied to measured optical signal parameters.
Accordingly, a method and system that enables efficient performance monitoring of an optical communications system remains highly desirable.